1. Field of the Invention
This invention relates to a method for producing a solid-liquid metal mixture in which non-dendritic primary solid particles are dispersed into the remaining liquid matrix (hereinafter referred to as a semi-solidified metal composition) through an electromagnetic induction agitating system and an apparatus used therefor.
2. Related Art Statement
As a method for the production of the semi-solidified metal composition, there are roughly known a mechanical agitating method and an electromagnetic induction agitating method. The electromagnetic induction agitating method (hereinafter referred to as an electromagnetic agitation simply) is poor in agitating efficiency as compared with the mechanical agitating method but is less restricted in the materials used in the apparatus and high in productivity. As a result, there have hitherto been proposed many improvements for the electromagnetic agitation.
In Japanese Patent Application Publication No. 61-7148 and No. 62-25464, there are disclosed a method of continuously or semicontinuously producing a metal slurry at a semi-solidified state through an electromagnetic agitation system and an apparatus used therefor.
In such a method, an electromagnetic agitation means producing a rotating magnetic field through a bipolar electric motor stator or the like is used and a mold provided with a cooling means is arranged inside thereof. Then molten metal is charged into the mold from above and cooled and agitated therein while being rotatably moved through the rotating magnetic field, whereby there is obtained a metal slurry of a semi-solidified state in which non-dendritic primary solid particles formed by breaking of dendrites are dispersed into the remaining liquid matrix.
In order to provide a metal slurry of good semi-solidified state, it is required to have strong cooling for forming sufficiently small solid particles and vigorous agitation strength for shearing dendrites. In the electromagnetic agitation system, however, the above two conditions are conflicting, so that it can not necessarily be said to satisfy the above conventional method and apparatus.
That is, there are the following problems in the conventional method and apparatus for the production of semi-solidified metal composition through an electromagnetic agitation system:
(1) In order to produce good semi-solidified metal compositions, it is necessary to give a vigorous agitation effect while cooling molten metal. If it is intended to conduct vigorous agitation through conventional electromagnetic agitation or high-speed rotating movement, a large eddy dent is created in the central portion of the rotating movement of molten metal through centrifugal force, while the level of the outer peripheral portion of molten metal becomes higher, consequently the scattering of molten metal from an upper part of a cooling agitation tank and the gas entrapment increases and stable operation is impossible. Therefore, the high-speed rotating movement or vigorous agitation effect can not be attained in the conventional electromagnetic agitation system.
(2) Although the central portion of molten metal is rotated at a high speed, the agitation effect is less and hence the agitation effect the horizontal section of molten metal becomes nonuniform. On the other hand, the rotating speed or agitation effect is dependent upon the viscosity of molten metal, so that as the apparent viscosity at the semi-solidified state becomes high, the agitation effect lowers and particularly the mixing effect is lost at the central portion and hence a risk of causing segregation becomes large.
(3) In order to produce the good semi-solidified metal composition, it is necessary to conduct strong cooling for forming sufficiently small solid particles. In the conventional electromagnetic agitation system, the internal volume of the cooling agitation tank is large with respect to the area of the inner wall or cooling wall thereof and the heat capacity of molten metal is large, so that the cooling rate can not be made fairly high due to heat generation of current produced through the rotating magnetic field.
On the other hand, when the strong cooling is carried out by using a water-cooled copper plate in the inner wall, the solidification shell adheres to the inner wall and gradually grows to largely reduce the magnetic flux of the rotating magnetic field, whereby the agitation effect is considerably decreased, so that the cooling strength in the inner wall is critical.
(4) In the conventional electromagnetic agitation system, the central portion of rotating movement of molten metal or the central portion of the cooling agitation tank forms a dead space for the production of the semi-solidified metal composition and is harmful and useless.